Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2018, 122, 13072−13081
Thermoplasmonic Effect of Surface-Enhanced Infrared Absorption in Vertical Nanoantenna Arrays Andrea Mancini,† Valeria Giliberti,‡ Alessandro Alabastri,§ Eugenio Calandrini,∥ Francesco De Angelis,∥ Denis Garoli,*,∥ and Michele Ortolani*,† †
Dipartimento di Fisica, Sapienza University of Rome, Piazzale Aldo Moro 5, 00185 Rome, Italy Center for Life NanoSciences, Istituto Italiano di Tecnologia (IIT), Viale Regine Elena 291, 00185 Rome, Italy § Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, United States ∥ Plasmon Nanotechnologies Department, Istituto Italiano di Tecnologia (IIT), 16163 Genoa, Italy
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‡
S Supporting Information *
ABSTRACT: Thermoplasmonics is a method for increasing temperature remotely using focused visible or infrared laser beams interacting with plasmonic nanoparticles. Here, local heating induced by mid-infrared quantum cascade laser illumination of vertical gold-coated nanoantenna arrays embedded into polymer layers is investigated by infrared nanospectroscopy and electromagnetic/thermal simulations. Nanoscale thermal hotspot images are obtained by a photothermal scanning probe microscopy technique with laser illumination wavelength tuned at the different plasmonic resonances of the arrays. Spectral analysis indicates that both Joule heating by the metal antennas and surface-enhanced infrared absorption (SEIRA) by the polymer molecules located in the apical hotspots of the antennas are responsible for thermoplasmonic resonances, that is, for strong local temperature increase. At odds with more conventional planar nanoantennas, the vertical antenna structure enables thermal decoupling of the hotspot at the antenna apex from the heat sink constituted by the solid substrate. The temperature increase was evaluated by quantitative comparison of data obtained with the photothermal expansion technique to the results of electromagnetic/thermal simulations. In the case of strong SEIRA by the CO bond of poly-methylmethacrylate at 1730 cm−1, for focused mid-infrared laser power of about 20 mW, the evaluated order of magnitude of the nanoscale temperature increase is of 10 K. This result indicates that temperature increases of the order of hundreds of K may be attainable with full mid-infrared laser power tuned at specific molecule vibrational fingerprints.
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example, catalysis,16,17 medical therapy,13,18 material synthesis,10,11 magnetic assisted recording,6 triggering of phase transitions,4,9 and thermophoresis.19 The transient electron temperature at the metal surface of antennas in the field-enhancement regions can be far higher than the steady-state metal lattice temperature,1 leading to socalled hot electron effects such as electron tunneling emission.16,17,20 Hot electron effects are distinct from the Joule heating effect, although they may ultimately contribute to local and global heating. Following the pioneering work of refs,21,24 in this work, we study a different path to thermoplasmonics based on e.m. energy dissipation by nonradiative decay of enhanced molecular dipole vibrations in the field-enhancement antenna hotspots. This mechanism of temperature increase is activated by laser illumination at substance-specific vibrational fingerprints in the mid-infrared (IR) range and belongs to the class of phenomena called surface-enhanced IR absorption (SEIRA).25,26 In this work, we
INTRODUCTION Thermoplasmonics has emerged as a new way of increasing temperature remotely by light beams,1,2 exploiting the Joule heating effect at optical frequencies in metal nanoparticles3−5 and nanoantennas.6−8 Thermoplasmonics has been explored up to now with two different aims: (i) to produce a strong, local temperature increase ΔT at the nanoscale, in close proximity of single plasmonic nanoparticles in which light excitation results in high local currents hence strong power dissipation in the metal4−7,9−13 or (ii) to increase the efficiency of radiation heating processes in large volumes or surfaces filled with both target molecules to be heated and plasmonic nanostructures acting as mediators of electromagnetic (e.m.) energy absorption for the entire system (global ΔT).3,8,12 The application of thermoplasmonic concepts has been carried out mostly with plasmonic nanoparticles randomly dispersed in solutions3,4 or with single plasmonic antennas;6,7 however, antenna arrays have also been considered14 because spatially coherent surface plasmon effects in periodic arrays may further enhance the absorption efficiency.15 Remotely light-activated local or global temperature increases can be of extreme importance in, for © 2018 American Chemical Society
Received: April 23, 2018 Published: May 3, 2018 13072
DOI: 10.1021/acs.jpcc.8b03808 J. Phys. Chem. C 2018, 122, 13072−13081
Article
The Journal of Physical Chemistry C
antennas was embedded in a weak IR-absorber polymer (AZ5214, spin-coating speed 1000−2000 rpm, final thickness 3.1−2.1 μm). A thin layer of a strong IR-absorber polymer was spin-coated on top of the vertical antennas [polymethyl methacrylate (PMMA) 950k, 2% solids in ethyl-lactate, spincoating speed 2000−3000 rpm, final thickness 400−100 nm], so as to fill the region where the apical antenna hotspots exist (i.e., those far away from the solid substrate acting as a heat sink). The mid-IR far-field response of the arrays is measured with a commercial Fourier-transform IR (FTIR) spectrometer (Bruker IFS66v/S) coupled either to a home-made variableangle reflection unit working with linearly polarized radiation on large-area samples (about 1 × 2 mm2) or to a reflective microscope (Bruker Hyperion), where quasi-unpolarized radiation is focused with a Cassegrain objective (incidence angle range of 10°−30° with respect to the surface normal) onto a 70 × 70 μm2 spot at the sample surface. A gold mirror is used as the reference to compute the absolute reflectance Rs,p(ω), and the extinction is calculated as 1 − Rs,p(ω) because transmission is zero, where the suffixes s and p refer to electric field polarization perpendicular and parallel to the incidence plane, respectively. Each spectrum is obtained as the average of 1024 interferometer scans at 4 cm−1 spectral resolution and the spectral range is 600−3000 cm−1. The nanoscale thermoplasmonic response of the system is investigated with an AFM operating in contact mode with the mechanical resonance-enhanced photothermal expansion technique (AFM-IR, Anasys Instruments NanoIR2 with top side illumination).28−30 The mid-IR light source is a tunable ECQCL (Daylight Solutions MIRcat-PX-B), with a continuous wavelength (wavenumber) range of 5.5−9.1 μm (1900−1100 cm−1) and accordable laser power range from 1 to 500 mW. To achieve full illumination of the probe tip, the laser beam impinges on the sample with a 70° angle with respect to the surface normal in p-polarization, leading to a Gaussian focal spot with elliptical footprint (major/minor axis with measured Gaussian width of 24/10 μm, see Supporting Information no. 2) centered on the AFM probe tip. A laser power of 19 mW at 1730 cm−1 gives a laser power density in the focus of 1.7 × 107 W/m2. The laser power varies within a factor of 3 in the tuning range of the EC-QCL. The AFM-IR photothermal expansion maps are obtained by monitoring the AFM probe cantilever deflection oscillations at the repetition rate of the EC-QCL, which is set to match the mechanical resonance of the cantilever (here, 220 kHz). The voltage scale VPSPD of the presented AFM-IR maps and spectra is the AFM positionsensitive photodetector signal component at the EC-QCL repetition rate of 220 kHz, measuring the laser-induced AFM cantilever deflection oscillations.30 The AFM topography maps are simultaneously recorded from the dc component of the deflection signal. The laser pulse duration is 260 ns, and the duty cycle is 6%. The spectral resolution is 2 cm−1. Uncoated silicon probe tips with slanted tip shafts (Nanosensors AdvancedTEC) were employed to avoid perturbation of nanoantenna fields (see Supporting Information no. 1). The e.m. simulations were performed using the commercial software COMSOL Multiphysics. Calculations were carried out by placing the antennas (described by a Drude−Lorentz electric permittivity44) on a metal half-space and filling the other half-space with a background medium (refractive index 1.6) up to the antenna height and placing air (refractive index 1) elsewhere. An absorbing PMMA layer, 100 nm thick, was
present nanoscale IR photothermal mapping of thermal hotspots in vertical antenna arrays embedded in polymer layers. In conjunction with e.m. and thermal simulations, the maps clearly show the existence of thermoplasmonic hotspots with strong temperature increase at the surface. Different from thermoplasmonic systems operating in the near-IR and visible ranges, where dissipation in the metal is the only relevant heat source, in the mid-IR, the contribution of molecular absorption can become important in determining the total temperature increase. In the context of mid-IR plasmonics, the novelty of our vertical antenna structures consists in the existence of an apical e.m. hotspot far from the solid substrate supporting the antennas, whereas in planar antenna structures, all hotspots are very close to the substrate that operates as the heat sink, hence preventing the exploitation of the heat produced in the hotspots. Also, vertical antenna arrays feature the high quality factor of the resonances, which permits having spectrally distinct plasmonic resonance peaks that can be designed to overlap with different IR vibrations of molecules. Organic molecule vibrations typically occur at moleculespecific frequencies in the “IR fingerprint” range 1000−2000 cm−1 (wavelengths between 5 and 10 μm), approximately covered by the tuning range of mid-IR external-cavity tunable quantum cascade lasers (EC-QCLs)27 featuring power density high enough for thermoplasmonic applications. Indeed, temperature increase effects induced by mid-IR EC-QCL illumination have been recently exploited for applications such as mid-IR absorption nanospectroscopy of few molecules,28−30 nonlinear optics in the mid-IR based on phase transitions of liquid crystals,31 and mapping of both fieldenhancement hotspots and currents in IR metamaterials.21−24,32,33 SEIRA has been long sought for in mid-IR plasmonic nanoantenna structures, however, it has been elusive up to now because of Fano interference phenomena34−40 that can prevent IR absorption enhancement while providing scattering enhancement.41,42 One way to observe true absorption enhancement phenomena is to look at thermoplasmonic effects instead of scattering enhancement. The present work on vertical nanoantenna structures constitutes a complete analysis of thermoplasmonic effects therein, not only caused by conventional Joule dissipation, where metal nanostructures are the sources of heat,1−8 but also caused by SEIRA, where the molecules are the sources of heat.21−24
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EXPERIMENTAL SECTION The vertical antennas consist in protrusions made of photoresist polymer spin-coated on a silicon wafer and then hardened by exposure to a focused ion beam. The nonexposed part of the film is removed in acetone. The resulting protrusions have a diameter of ∼200 nm and a height H equal to the initial polymer film thickness. The fabrication process is entirely described in ref 43. The antenna arrays, including the flat silicon wafer surface, were coated conformally with a 80 nm thick evaporated gold film (thicker than 3 times the skin depth of gold in the mid-IR44), closing the access to the hollow cavity inside the protrusion and leading to a final gold-coated vertical rod antenna structure with diameter dant ≈ 360 nm.43,45 The samples used in this work are square periodic arrays of vertical nanoantennas with H ranging from 2.2 to 2.7 μm and pitch P equal to 3.0, 3.5, or 4.0 μm. After fabrication, the vertical antennas were embedded up to their top in a spin-coated polymer bilayer film, so as to allow their nanoscale investigation by atomic force microscopy (AFM). The entire body of the 13073
DOI: 10.1021/acs.jpcc.8b03808 J. Phys. Chem. C 2018, 122, 13072−13081
Article
The Journal of Physical Chemistry C located between air and top antenna plane. The fabricated and experimentally characterized samples were modeled as infinite three-dimensional (3D) arrays of antennas where Floquet periodic conditions were set in the planar directions to define the rectangular unit cells. Input radiation was accounted for by setting e.m. ports at the top surface of the modeled domain, on the interior side of the perfect matching layer, setting an angle of incidence θinc. The incident radiation was represented by a linearly polarized plane wave, the absorption and scattering spectra were obtained, and the field-enhancement maps were then calculated at the plasmonic resonance frequencies derived from the simulated spectra.
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RESULTS AND DISCUSSION In Figure 1a,b SEM images of the samples before the polymer embedding procedure are shown. The antennas are constituted by gold-coated vertical cylinders protruding from a flat surface, also gold-coated. The geometrical parameters (height H and array pitch P) were chosen on the basis of previous work45 so as to overlap the plasmonic resonance frequencies in the mid-IR range with the molecular vibration frequencies of the two different polymers [azobenzene (AZ) and PMMA] embedding the vertical antennas. As a further fine-tuning parameter for obtaining better frequency overlaps, the vertical antennas were also fabricated with a tilt angle θtilt = 10° or 20° with respect to the surface-normal direction. The tilt angle acts as a small perturbation of the plasmonic modes of the array, whose resonance frequency is mainly set by P and H and by the incidence angle θinc of the illuminating IR beam. In Figure 1c, the plasmonic mode structure of the bare samples is investigated by variable incidence angle FTIR reflection spectroscopy. The p-polarized reflectivity Rp (radiation electric field partly parallel to the antenna axis) shows dips at plasmonic mode frequencies, corresponding to extinction peaks, not seen in the s-polarized reflectivity Rs (radiation electric field orthogonal to the antenna axis). Starting form almost-normal incidence (θinc = 10°), two modes appear in the Rp spectra, which we call bright modes at frequencies ω1b and ω2b almost independent of the incidence angle. At strongly non-normal incidence, a mode with angular dispersion appears, activated by asymmetry of the illumination direction. Therefore, we call this mode the asymmetric mode. The bright modes have an intrinsic dipole moment related to the antenna structure, whereas the asymmetric mode is a “spoof” surface plasmon mode related to the periodic array structure (see Supporting Information no. 5). The vertical antenna arrays were then embedded in polymers and investigated by AFM-IR,28−30 a technique suitable for mapping the local photothermal expansion in the nanoscale antenna hotspots (see an example in inset of Figure 1a).21−24,32,33,46 E.m. absorption maps and temperature maps have been determined by 3D numerical simulations, reported in Figure 1d−f for a typical resonance condition where both the plasmonic nanostructure and the top molecular layer (PMMA) separately present a maximum in the e.m. response. In Figure 1d, the e.m. absorption is plotted in logarithmic scale using the absorption A0 of the same PMMA layer without antennas as the reference value. It is clear that both the vibrational absorption of the PMMA molecules and the Ohmic losses of the metal antenna can generate comparable amounts of heat. For the PMMA molecules located in the apical antenna hotspot, the absorption is locally enhanced by several orders of magnitude (SEIRA effect), as highlighted in Figure 1e, where
Figure 1. (a,b) Scanning electron micrographs of an array of vertical antennas, with 10° tilt angle in (a) and with zero tilt angle in (b). Insets of panel (a) AFM topography and AFM-IR photothermal expansion image of a portion of the array embedded into a polymer bilayer. (c) Variable incidence angle reflectance spectra of the antenna arrays. Plasmonic resonances appear in the p-polarized spectra covering the entire mid-IR vibrational absorption range of organic molecules. (d) e.m. simulations at θinc = 70° of the local e.m. absorption in a sample geometry giving a plasmonic resonance that matches the frequency of the strong CO stretching vibration of PMMA. (e) Same as (d) but with different color scale, and map limited to the PMMA layer, to make SEIRA in the antenna hotspot more evident. (f) Thermal simulations of the temperature reached by each point at the end of the mid-IR laser pulse for 19 mW of laser illumination power, using the absorption in (d) as the heat source. The air layer at the top of the cell and the Si substrate at the bottom are set as heat sinks at 293 K.
the same simulation of Figure 1d is presented with shifted color scale. In Figure 1f, the resulting temperature map is shown: because of combined effect of the large distance of the apical hotspot from the substrate heat sink and of high thermal conductivity of gold, the ΔT along the antenna shaft is much smaller than the ΔTmax of the PMMA molecules in the thermal hotspot, reaching up to ΔTmax ≈ 20 K for 19 mW of laser illumination power at the CO stretching vibration of PMMA. Air molecules in the vicinity of the thermal hotspot are also heated considerably, as already reported for vertical antenna arrays.8 Elsewhere, ΔT is below 3 K. 13074
DOI: 10.1021/acs.jpcc.8b03808 J. Phys. Chem. C 2018, 122, 13072−13081
Article
The Journal of Physical Chemistry C
Looking at the molecular absorption spectra of the embedding polymers reported in Figure 2b, AZ can be considered as a weak IR-absorber, while PMMA is a wellknown strong IR-absorber at the three main molecule absorption bands around 1150−1250, 1450, and 1730 cm−1 corresponding to C−O−C stretching, C−H bending, and C O (carbonyl) stretching vibrations, respectively. Frequency overlap of the PMMA vibrations with the plasmonic modes of the vertical antenna arrays (a necessary requirement for SEIRA) is summarized in Figure 2 by color dots. In Figure 2a, a strong thermoplasmonic peak is observed in the AFM-IR spectrum at 1850 cm−1 (red dot), where there is no molecular vibration, and it is therefore attributed to Joule heating at the second bright plasmonic mode at ω2b. The small offset between the peak frequency in 1 − Rp(ω) and that in the AFM-IR spectrum around 1850 cm−1 is due to the difference in resonance frequencies between far-field and near-field detection of the plasmonic antenna resonance.52 Besides the bright mode peak at 1850 cm−1, in Figure 2a the asymmetric plasmonic mode is observed in the AFM-IR spectrum at 1250 cm−1 (green dot), with a clear counterpart in the corresponding 1 − Rp(ω). The peak at 1250 cm−1 also corresponds to the C−O−C stretching vibration of PMMA, leading to an increase of the molecular dipole absorption strength (SEIRA) if compared for example, with the nonenhanced vibration at 1730 cm−1 (pink dot). Interestingly, in sample sA at 1250 cm−1, one directly observes the thermoplasmonic effect of SEIRA in the AFM-IR spectrum,21−24 and not just its consequences in the reflection spectrum, typically resulting in asymmetric Fano lineshapes.26 Note that the high quality factor of plasmonic resonances in vertical antenna arrays allows for the visualization of both modes separately in the same AFM-IR spectrum. The photothermal expansion maps at 1850 cm−1 (Joule heating only) and at 1250 cm−1 (both SEIRA and Joule heating) are reported in Figure 2c for sample sA: therein, nanothermal hotspots are clearly detected.21−24,32,33,46 In the control experiments at the molecular absorption frequencies of 1450 and 1730 cm−1 (black and pink dots, respectively), in the absence of plasmonic field enhancement, the maps of Figure 2c do not show any thermal hotspot because heat is generated everywhere on the sample surface by direct molecular absorption (Beer−Lambert). The thermal hotspots seen in the AFM-IR maps of Figure 2c at 1850 and 1250 cm−1 display half-width half-maximum diameters around 600 nm. In vertical nanoantennas, the field-enhancement regions (i.e. the e.m. hotspots) are only slightly broader than dant ≈ 360 nm.40,43,45 The difference between the hotspot size measured with AFMIR imaging (600 nm) and the e.m. hotspot size (
